1. Field of the Invention
The present invention relates to a plating method and a plating apparatus, and more particularly to a plating method and a plating apparatus for filling a metal such as copper (Cu) or the like into fine interconnection patterns (trenches) on a semiconductor substrate.
2. Description of the Related Art
Aluminum or an aluminum alloy has generally been used as a material for forming interconnect circuits on semiconductor substrates. As integrated density has increased in recent years, there is a demand for usage of a material having a higher conductivity as an interconnect material. It has been proposed to plate a substrate having interconnect pattern trenches thereon to fill the trenches with copper or its alloy.
There are known various processes including CVD (chemical vapor deposition), sputtering, and the like to fill interconnect pattern trenches with copper or its alloy. However, the CVD process is costly for forming copper interconnections, and the sputtering process fails to embed copper or its alloy in interconnect pattern trenches when the interconnect pattern trenches have a high aspect ratio, i.e., a high ratio of depth to width. A plating process is most effective to deposit a metal layer of copper or its alloy on a substrate to form copper interconnections thereon.
Various processes are available for plating semiconductor substrates with copper. These include a process of immersing a substrate in a plating liquid held at all times in a plating tank, referred to as a cup-type or dipping-type process; a process of holding a plating liquid in a plating tank only when a substrate, to be plated, is supplied to the plating tank; an electric plating process for plating a substrate with a potential difference; and an electroless plating process for plating a substrate with no potential difference.
In carrying out filling of fine interconnect patterns with copper by electric copper-plating using a copper sulfate solution as a plating liquid, it is required to perform a plating process with high throwing power and high leveling properties. With a view to meeting this requirement, it is generally known to add to the plating liquid a compound called an additive.
Such an additive, generally in use, includes:
sulfur compounds called xe2x80x9ccarrierxe2x80x9d, which grow crystal nuclei all over a plated surface, thereby promoting deposition of finer particles;
polymers which increase over-voltage of copper deposition, thereby enhancing throwing power; and
nitrogen compounds called xe2x80x9clevelerxe2x80x9d, which adhere to convex portions, where plating preferentially grows, to thereby increase over-voltage and retard copper deposition at the convex portions, thereby providing a flat plated layer.
However, when filling fine interconnect patterns with copper by electric copper-plating is conducted by using a plating liquid which, due to use of the above additives, has enhanced throwing power and leveling properties, there occurs a phenomena that a film thickness of an interconnection region of a substrate becomes thicker than a film thickness of a non-interconnection region. Unevenness in film thickness is not a problem in filling the interconnection region with copper; however, unevenness makes it difficult to obtain a flat surface by performing later CMP (chemical mechanical polishing) processing.
A plating treatment of a substrate for filling interconnect pattern trenches with a metal, such as copper or its alloy, may be carried out by using a plating apparatus as shown in FIG. 30. As shown in FIG. 30, a substrate W and an anode 302 are disposed in parallel, facing each other, in a plating tank 301 accommodating a plating liquid 300. Plating is conducted by flowing a plating current i between the substrate W and the anode 302. A film thickness h of a plated film formed at a certain point on a surface of the substrate W is proportional to a product of a plating current value and energization time. The plating current value in FIG. 30 is defined by the following formula (1):
i=E/(R1+R2+R3+R4)xe2x80x83xe2x80x83(1)
In the above formula (1), E represents power source voltage, R1 anodic polarization resistance, R2 resistance of the plating liquid 300, R3 substrate (cathodic) polarization resistance, and R4 sheet resistance of the substrate W at the certain point. The anodic polarization resistance R1 and the substrate polarization resistance R3 are interfacial resistances of the anode 302 and of the substrate W, respectively, and change with concentration of an additive or of the plating liquid. The resistance R2 of the plating liquid 300 is proportional to a distance between the anode 302 and the substrate (cathode) W.
An electric supply to the substrate W is made via a cathode electrode 303 which is generally connected to a peripheral end of the substrate W. Accordingly, the sheet resistance R4 at a point increases as a distance from the peripheral end of the substrate W increases, i.e., as the point comes near to center P of the substrate W. Therefore, the plating current value on an inner central side of the substrate W is smaller than that on an outer peripheral side (see the above formula (1)), whereby it is likely that film thickness becomes smaller on the inner central side as compared to the outer peripheral side. There has thus been a problem in conventional plating apparatuses that a plated film having a uniform film thickness over an entire substrate surface is difficult to form. Especially when an LSI interconnection is formed by plating, a small thickness, generally 50-200 nm, of a seed layer of the substrate (Si substrate) makes the sheet resistance R4 considerably larger. Such a large sheet resistance R4 has a larger influence on film thickness.
The present invention has been made in view of the above drawbacks in the related art. It is therefore a first object of the present invention to provide a plating method and a plating apparatus which can attain embedding of copper into fine interconnect patterns with use of a plating liquid having high throwing power and leveling properties, and which can make film thickness of a plated film substantially equal between an interconnection region and a non-interconnection region, thereby facilitating later CMP processing.
It is a second object of the present invention to provide a plating apparatus and a plating method which can form a plated film having a more uniform film thickness over an entire surface of a substrate.
In order to achieve the first object, the present invention provides a plating method, comprising: filling a plating liquid containing metal ions and an additive into a plating space formed between a substrate and an anode disposed closely to the substrate so as to face the substrate; and changing concentration of an additive in the plating liquid filled into the plating space during a plating process.
In the course of plating of a substrate, concentration of an additive in a plating liquid filled into a plating space formed between the substrate and an anode gradually decreases with duration of the plating due to take-in of the additive within deposited metal film and oxidation degradation at the anode. The change of additive concentration is larger in cases where {circle around (1)} plating of a substrate is by a close-to-anode plating where an amount of plating liquid itself is small, {circle around (2)} introduction of a plating liquid into the plating space is conducted only before plating, and not conducted during plating (batch-wise introduction), and {circle around (3)} a plating liquid is introduced intermittently during plating. The concentration change of the plating liquid is larger when, during a plating process, an additional solution or a plating liquid containing a different concentration of additive is separately introduced into the plating space with a separate liquid introduction device.
By thus changing additive concentration of a plating liquid filled into a plating space during a plating process, unevenness in plated film thickness between an interconnection region and a non-interconnection region is reduced or corrected.
It is not fully clarified by what mechanism a difference in film thickness between the interconnection and non-interconnection regions is corrected by making a change in the additive concentration, during the plating process, of the plating liquid filled into the plating space. Anyway, in general, the difference in film thickness can be effectively corrected when concentration of an additive decreases during the plating process; when concentration of a particular additive, especially a plating-promoting additive called xe2x80x9cbrightenerxe2x80x9d, is set at a high value; or when content of an additive is significantly lowered by, for example, adsorption removal of the additive. The film-thickness difference in question is considered to be produced at a middle or later stage of the plating process when filling metal into fine interconnect trenches has almost been completed. Accordingly, making a change in additive concentration of a plating liquid at a middle or later stage of plating is more effective than that at an initial stage when filling metal into interconnect trenches is in progress.
Concentration of an additive in a plating liquid can be adjusted by intermittently supplying the plating liquid into a plating space.
Additive concentration can also be adjusted by supplementary addition of the additive to a plating space, or by removal of the additive in a plating liquid.
The present invention also provides a plating apparatus, comprising: a substrate holder for holding a substrate so that a current can flow from a cathode to the substrate; an anode opposed to the substrate held by the substrate holder; and a plating liquid introducing device for introducing a plating liquid into a plating space formed between the substrate and the anode during a batch process or an intermittent process.
This apparatus can perform a plating treatment while changing concentration of an additive in a plating liquid filled into the plating space.
A plating liquid impregnation material may be provided in the plating space. The plating liquid impregnation material, e.g. synthetic fibers can adsorb and remove a particular additive component, e.g. a leveler, and thus is effective for reducing leveler concentration of a plating liquid.
Further, the plating apparatus may be provided with a liquid introducing device for introducing into the plating space a liquid having a different additive concentration from that in the above plating liquid. The addition of the liquid (solution or plating liquid) having the different additive concentration makes it possible to arbitrarily control, during a plating process, change of additive concentration in plating liquid filled into the plating space formed between the substrate and the anode. For example, addition of a liquid having a higher leveler concentration, during a plating process, can correct a film-thickness difference.
The plating apparatus may also be provided with a temperature adjusting device for adjusting a temperature of plating liquid in the plating space. Since adsorption reaction, which occurs on the above plating liquid impregnation material is highly temperature-dependent, use of a higher plating liquid temperature generally increases adsorption capacity of the plating liquid impregnation material.
In order to achieve the second object, the present invention provides a plating apparatus, comprising: a substrate holder for holding a substrate so that a current can flow from a cathode to the substrate; an anode opposed to the substrate held by the substrate holder; and a moving device for moving a portion of the substrate facing the anode in such a state that an inner central portion of a surface of the substrate faces the anode for a longer time than does an outer peripheral portion of the surface of the substrate.
This plating apparatus can make energization time of a plating current to the inner central portion of the surface of the substrate longer than energization time of the plating current to the outer peripheral portion of the surface of the substrate, thereby making products of electric current values and energization times of the electric current, at various points of the substrate, equal over an entire surface of the substrate. This enables formation of a plated film having a uniform film thickness over the entire surface of the substrate.
The moving device may comprise a substrate-rotating device for rotating the substrate, an anode-rotating device for rotating the anode, or an anode-translating device for translating the anode.
The present invention also provides another plating apparatus comprising: a substrate holder for holding a substrate so that a current can flow from a cathode to the substrate; and an anode opposed to the substrate held by the substrate holder, wherein a distance between the anode and an inner central portion of a surface of the substrate is smaller than a distance between the anode and an outer peripheral portion of the surface of the substrate.
This apparatus can make resistance of a plating liquid at the inner central portion of the substrate smaller than that at the outer peripheral portion of the surface of the substrate, thereby making an electric current value more equal at the inner central portion of the surface of the substrate to that at the outer peripheral portion of the surface of the substrate, whereby film thickness of plated film formed on the substrate can be made uniform over an entire surface of the substrate.
The present invention further provides a yet another plating apparatus comprising: a substrate holder for holding a substrate so that a current can flow from a cathode to the substrate; an anode opposed to the substrate held by the substrate holder; and a distance changing device for changing a distance between the substrate and the anode after initiation of plating.
At initiation of plating, a potential gradient on an inner central side of a surface of the substrate is higher than a potential gradient on an outer peripheral side of the surface of the substrate, whereby a larger amount of plated film is formed on the inner central side of the surface of the substrate. This situation can be reversed according to this apparatus, by later making a distance between the substrate and the anode larger. As a result, a plated film having a uniform film thickness over an entire surface of the substrate can be obtained.
The present invention also provides a plating method, comprising: disposing a substrate and an anode in such a state that the substrate faces the anode; flowing a current between the substrate and the anode while supplying a plating liquid therebetween; and moving a portion of the substrate facing the anode in such a state that an inner central portion of a surface of the substrate faces the anode for a longer time than does an outer peripheral portion of the surface of the substrate.
The present invention also provides another plating method, comprising: disposing a substrate and an anode in a state that the substrate faces the substrate; and flowing a current between the substrate and the anode while supplying a plating liquid therebetween, wherein a distance between the anode and an inner central portion of a surface of the substrate is smaller than a distance between the anode and an outer peripheral portion of the surface of the substrate.
The present invention further provides yet another plating method, comprising: disposing a substrate and an anode in a state that the substrate faces the anode; flowing a current between the substrate and the anode while supplying a plating liquid therebetween; and changing a distance between the substrate and the anode after initiation of plating.